Copyright Notice

Abstract

This document explains why mass unsolicited electronic mail messages
are harmful in the Internetworking community. It gives a set of
guidelines for dealing with unsolicited mail for users, for system
administrators, news administrators, and mailing list managers. It
also makes suggestions Internet Service Providers might follow.

1. Introduction

The Internet's origins in the Research and Education communities
played an important role in the foundation and formation of Internet
culture. This culture defined rules for network etiquette
(netiquette) and communication based on the Internet's being
relatively off-limits to commercial enterprise.

This all changed when U.S. Government was no longer the primary
funding body for the U.S. Internet, when the Internet truly went
global, and when all commercial enterprises were allowed to join what
had been strictly research networks. Internet culture had become
deeply embedded in the protocols the network used. Although the
social context has changed, the technical limits of the Internet
protocols still require a person to enforce certain limits on
resource usage for the 'Net to function effectively. Strong
authentication was not built into the News and Mail protocols. The
only thing that is saving the Internet from congestion collapse is
the voluntary inclusion of TCP backoff in almost all of the TCP/IP
driver code on the Internet. There is no end-to-end cost accounting
and/or cost recovery. Bandwidth is shared among all traffic without
resource reservation (although this is changing).

Unfortunately for all of us, the culture so carefully nurtured
through the early years of the Internet was not fully transferred to
all those new entities hooking into the bandwidth. Many of those
entities believe they have found a paradise of thousands of potential
customers each of whom is desperate to learn about stunning new
business opportunities. Alternatively, some of the new netizens
believe all people should at least hear about the one true religion
or political party or process. And some of them know that almost no
one wants to hear their message but just can't resist how inexpensive
the net can be to use. While there may be thousands of folks
desperate for any potential message, mass mailings or Netnews
postings are not at all appropriate on the 'Net.

This document explains why mass unsolicited email and Netnews posting
(aka spam) is bad, what to do if you get it, what webmasters,
postmasters, and news admins can do about it, and how an Internet
Service Provider might respond to it.

2. What is Spam*?

The term "spam" as it is used to denote mass unsolicited mailings or
netnews postings is derived from a Monty Python sketch set in a
movie/tv studio cafeteria. During that sketch, the word "spam" takes
over each item offered on the menu until the entire dialogue consists
of nothing but "spam spam spam spam spam spam and spam." This so
closely resembles what happens when mass unsolicited mail and posts
take over mailing lists and netnews groups that the term has been
pushed into common usage in the Internet community.

When unsolicited mail is sent to a mailing list and/or news group it
frequently generates more hate mail to the list or group or apparent
sender by people who do not realize the true source of the message.
If the mailing contains suggestions for removing your name from a
mailing list, 10s to 100s of people will respond to the list with
"remove" messages meant for the originator. So, the original message
(spam) creates more unwanted mail (spam spam spam spam), which
generates more unwanted mail (spam spam spam spam spam spam and
spam). Similar occurrences are perpetrated in newsgroups, but this
is held somewhat in check by "cancelbots" (programs which cancel
postings) triggered by mass posting. Recently, cancelbots have grown
less in favor with those administering News servers since the
cancelbots are now generating the same amount of traffic as spam.
Even News admins are beginning to use filters, demonstrating that
spam spam spam spam spam spam and spam is a monumental problem.

3. Why Mass Mailing is Bad

In the world of paper mail we're all used to receiving unsolicited
circulars, advertisements, and catalogs. Generally we don't object
to this - we look at what we find of interest, and we discard/recycle
the rest. Why should receiving unsolicited email be any different?

The answer is that the cost model is different. In the paper world,
the cost of mailing is borne by the sender. The sender must pay for
the privilege of creating the ad and the cost of mailing it to the
recipient. An average paper commercial mailing in the U.S. ends up
costing about $1.00 per addressee. In the world of electronic
communications, the recipient bears the majority of the cost. Yes,
the sender still has to compose the message and the sender has to pay
for Internet connectivity. However, the recipient ALSO has to pay
for Internet connectivity and possibly also connect time charges and
for disk space. For electronic mailings the recipient is expected to
help share the cost of the mailing. Bulk Internet mail from the U.S.
ends up costing the sender only about 1/100th of a cent per address;
or FOUR ORDERS of magnitude LESS than bulk paper mailings!

Of course, this cost model is very popular with those looking for
cheap methods to get their message out. By the same token, it's very
unpopular with people who have to pay for their messages just to find
that their mailbox is full of junk mail. Neither do they appreciate
being forced to spend time learning how to filter out unwanted
messages. Consider this: if you had to pay for receiving paper mail
would you pay for junk mail?

Another consideration is that the increase in volume of spam will
have an impact on the viability of electronic mail as a
communications medium. If, when you went to your postal mail box you
found four crates of mail, would you be willing to search through the
crates for the one or two pieces of mail which were not advertising?
Spam has a tremendous potential to create this scenario in the
electronic world.

Frequently spammers indulge in unethical behavior such as using mail
servers which allow mail to be relayed to send huge amounts of
electronic solicitations. Or they forge their headers to make it
look as if the mail originates from a different domain. These people
don't care that they're intruding into a personal or business mailbox
nor do they care that they are using other people's resources without
compensating them.

The huge cost difference has other bad effects. Since even a very
cheap paper mailing is going to cost tens of (U.S.) cents there is a
real incentive to send only to those really likely to be interested.

So paper bulk mailers frequently pay a premium to get high quality
mailing lists, carefully prune out bad addresses and pay for services
to update old addresses. Bulk email is so cheap that hardly anyone
sending it bothers to do any of this. As a result, the chance that
the receiver is actually interested in the mail is very, very, very
low.

As of the date of this document, it is a daily event on the Internet
for a mail service to melt-down due to an overload of spam. Every
few months this happens to a large/major/regional/
national/international service provider resulting in denial of or
severe degradation of service to hundreds of thousands of users.
Such service degradations usually prompt the providers to spend
hundreds of thousands of dollars upgrading their mail service
equipment just because of the volume of spam. Service providers pass
those costs on to customers.

Doesn't the U.S. Constitution guarantee the ability to say whatever
one likes? First, the U.S. Constitution is law only in the U.S., and
the Internet is global. There are places your mail will reach where
free speech is not a given. Second, the U.S. Constitution does NOT
guarantee one the right to say whatever one likes. In general, the
U.S. Constitution refers to political freedom of speech and not to
commercial freedom of speech. Finally, and most importantly, the U.S.
Constitution DOES NOT guarantee the right to seize the private
property of others in order to broadcast your speech. The Internet
consists of a vast number of privately owned networks in voluntary
cooperation. There are laws which govern other areas of electronic
communication, namely the "junk fax" laws. Although these have yet
to be applied to electronic mail they are still an example of the
"curbing" of "free speech." Free speech does not, in general,
require other people to spend their money and resources to deliver or
accept your message.

Most responsible Internet citizens have come to regard unsolicited
mail/posts as "theft of service". Since the recipient must pay for
the service and for the most part the mail/posts are advertisements
of unsolicited "stuff" (products, services, information) those
receiving it believe that the practice of making the recipient pay
constitutes theft.

The crux of sending large amounts of unsolicited mail and news is not
a legal issue so much as an ethical one. If you are tempted to send
unsolicited "information" ask yourself these questions: "Whose
resources is this using?" "Did they consent in advance?" "What
would happen if everybody (or a very large number of people) did
this?" "How would you feel if 90% of the mail you received was
advertisements for stuff you didn't want?" "How would you feel if 95%

of the mail you received was advertisements for stuff you didn't
want?" "How would you feel if 99% of the mail you received was
advertisements for stuff you didn't want?"

Although numbers on the volume and rate of increase of spam are not
easy to find, seat-of-the-pants estimates from the people on spam
discussion mailing lists [1] indicate that unsolicited mail/posts
seems to be following the same path of exponential growth as the
Internet as a whole [2]. This is NOT encouraging, as this kind of
increase puts a strain on servers, connections, routers, and the
bandwidth of the Internet as a whole. On a per person basis,
unsolicited mail is also on the increase, and individuals also have
to bear the increasing cost of increasing numbers of unsolicited and
unwanted mail. People interested in hard numbers may want to point
their web browsers to
http://www.techweb.com/se/directlink.cgi?INW19980504S0003 where
Internet Week reports what spam costs.

Finally, sending large volumes of unsolicited email or posting
voluminous numbers of Netnews postings is just plain rude. Consider
the following analogy: Suppose you discovered a large party going on
in a house on your block. Uninvited, you appear, then join each
group in conversation, force your way in, SHOUT YOUR OPINION (with a
megaphone) of whatever you happen to be thinking about at the time,
drown out all other conversation, then scream "discrimination" when
folks tell you you're being rude.

To continue the party analogy, suppose instead of forcing your way
into each group you stood on the outskirts a while and listened to
the conversation. Then you gradually began to add comments relevant
to the discussion. Then you began to tell people your opinion of the
issues they were discussing; they would probably be less inclined to
look badly on your intrusion. Note that you are still intruding.
And that it would still be considered rude to offer to sell products
or services to the guests even if the products and services were
relevant to the discussion. You are in the wrong venue and you need
to find the right one.

Lots of spammers act as if their behavior can be forgiven by
beginning their messages with an apology, or by personalizing their
messages with the recipient's real name, or by using a number of
ingratiating techniques. But much like the techniques used by Uriah
Heep in Dickens' _David Copperfield_, these usually have an effect
opposite to the one intended. Poor excuses ("It's not illegal,"
"This will be the only message you receive," "This is an ad," "It's
easy to REMOVE yourself from our list") are still excuses. Moreover,
they are likely to make the recipient MORE aggravated rather than
less aggravated.

In particular, there are two very severe problems with believing that
a "remove" feature to stop future mail helps: (1) Careful tests have
been done with sending remove requests for "virgin" email accounts
(that have never been used anywhere else). In over 80% of the cases,
this resulted in a deluge of unsolicited email, although usually from
other sources than the one the remove was sent to. In other words,
if you don't like unsolicited mail, you should think carefully before
using a remove feature because the evidence is that it will result in
more mail not less. (2) Even if it did work, it would not stop lots
of new unsolicited email every day from new businesses that hadn't
mailed before.

4a. ACK! I've Been Spammed - Now What?

It's unpleasant to receive mail which you do not want. It's even
more unpleasant if you're paying for connect time to download it.
And it's really unpleasant to receive mail on topics which you find
offensive. Now that you're good and mad, what's an appropriate
response?

First, you always have the option to delete it and get on with your
life. This is the easiest and safest response. It does not
guarantee you won't get more of the same in the future, but it does
take care of the current problem. Also, if you do not read your mail
on a regular basis it is possible that your complaint is much too
late to do any good.

Second, consider strategies that take advantage of screening
technology. You might investigate technologies that allow you to
filter unwanted mail before you see it. Some software allows you to
scan subject lines and delete unwanted messages before you download
them. Other programs can be configured to download portions of
messages, check them to see if they are advertising (for example) and
delete them before the whole message is downloaded.

Also, your organization or your local Internet Service Provider may
have the ability to block unwanted mail at their mail relay machines
and thus spare you the hassle of dealing with it at all. It is worth
inquiring about this possibility if you are the victim of frequent
spam.

Your personal mailer software may allow you to write rules defining
what you do and do not wish to read. If so, write a rule which sends
mail from the originator of the unwanted mail to the trash. This
will work if one sender or site repeatedly bothers you. You may also
consider writing other rules based on other headers if you are sure
the probability of them being activated for non-spam is low enough.
That way, although you may still have to pay to download it, you
won't have to read it!

Third, you may consider sending the mail back to the originator
objecting to your being on the mailing-list; however, we recommend
against this. First, a lot of spammers disguise who they are and
where their mail comes from by forging the mail headers. Unless you
are very experienced at reading headers discovering the true origin
of the mail will probably prove difficult. Although you can engage
your local support staff to help you with this, they may have much
higher priorities (such as setting up site-wide filters to prevent
spam from entering the site). Second, responding to this email will
simply verify your address as valid and make your address more
valuable for other (ab)uses (as was mentioned above in Section 3).
Third, even if the two previous things do not happen, very probably
your mail will be directed to the computer equivalent of a black hole
(the bit-bucket).

As of the writing of this document, there are several pieces of
pending legislation in several jurisdictions about the sending of
unsolicited mail and also about forging headers. If forging of
headers should become illegal, then responding to the sender is less
risky and may be useful.

Certainly we advocate communicating to the originator (as best as you
can tell) to let them know you will NOT be buying any products from
them as you object to the method they have chosen to conduct their
business (aka spam). Most responses through media other than
electronic mail (mostly by those who take the time to phone included
"800" (free to calling party in the U.S.) phone numbers) have proved
somewhat effective. You can also call the business the advertisement
is for, ask to speak to someone in authority, and then tell them you
will never buy their products or use their services because their
advertising mechanism is spam.

Next, you can carbon copy or forward the questionable mail messages
or news postings to your postmaster. You can do this by sending mail
"To: Postmaster@your-site.example." Your postmaster should be an
expert at reading mail headers and will be able to tell if the
originating address is forged. He or she may be able to pinpoint the
real culprit and help close down the site. If your postmaster wants
to know about unsolicited mail, be sure s/he gets a copy, including
headers. You will need to find out the local policy and comply.

*** IMPORTANT ***

Wherever you send a complaint, be sure to include the full headers
(most mail and news programs don't display the full headers by
default). For mail it is especially important to show the
"Received:" headers. For Usenet news, it is the "Path:" header.
These normally show the route by which the mail or news was
delivered. Without them, it's impossible to even begin to tell where
the message originated. See the appendix for an example of a mail
header.

There is lively and ongoing debate about the validity of changing
one's email address in a Web Browser in order to have Netnews posts
and email look as if it is originating from some spot other than
where it does originate. The reasoning behind this is that web email
address harvesters will not be getting a real address when it
encounters these. There is reason on both sides of this debate: If
you change your address, you will not be as visible to the
harvesters, but if you change your address, real people who need to
contact you will be cut off as well. Also, if you are using the
Internet through an organization such as a company, the company may
have policies about "forging" addresses - even your own! Most people
agree that the consequences of changing your email address on your
browser or even in your mail headers is fairly dangerous and will
nearly guarantee your mail goes into a black hole unless you are very
sure you know what you are doing.

Finally, DO NOT respond by sending back large volumes of unsolicited
mail. Two wrongs do not make a right; do not become your enemy; and
take it easy on the network. While the legal status of spam is
uncertain, the legal status (at least in the U.S.) of a "mail bomb"
(large numbers and/or sizes of messages to the site with the intent
of disabling or injuring the site) is pretty clear: it is criminal.

There is a web site called "www.abuse.net" which allows you to
register, then send your message to the name of the "offending-
domain@abuse.net," which will re-mail your message to the best
reporting address for the offending domain. The site contains good
tips for reporting abuse netnews or email messages. It also has some
automated tools that you may download to help you filter your
messages. Also check CIAC bulletin I-005 at:

http://ciac.llnl.gov/ciac/bulletins/i-005c.shtml

or at:

http://spam.abuse.net/spam/tools/mailblock.html.

Check the Appendix for a detailed explanation of tools and
methodology to use when trying to chase down a spammer.

4b. There's a Spam in My Group!

Netnews is also subject to spamming. Here several factors help to
mitigate against the propagation of spam in news, although they don't
entirely solve the problem. Newsgroups and mailing lists may be
moderated, which means that a moderator approves all mail/posts. If
this is the case, the moderator usually acts as a filter to remove
unwanted and off-topic posts/mail.

In Netnews there are programs which detect posts which have been sent
to multiple groups or which detect multiple posts from the same
source to one group. These programs cancel the posts. While these
work and keep unsolicited posts down, they are not 100% effective and
spam in newsgroups seems to be growing at an even faster rate than
spam in mail or on mailing lists. After all, it's much easier to
post to a newsgroup for which there are thousands of readers than it
is to find individual email addresses for all those folks. Hence the
development of the "cancelbots" (sometimes called "cancelmoose") for
Netnews groups. Cancelbots are triggered when one message is sent to
a large number of newsgroups or when many small messages are sent
(from one sender) to the same newsgroup. In general these are tuned
to the "Breidbart Index" [3] which is a somewhat fuzzy measure of the
interactions of the number of posts and number of groups. This is
fuzzy purposefully, so that people will not post a number of messages
just under the index and still "get away with it." And as noted
above, the cancel messages have reached such a volume now that a lot
of News administrators are beginning to write filters rather than
send cancels. Still spam gets through, so what can a concerned
netizen do?

If there is a group moderator, make sure s/he knows that off-topic
posts are slipping into the group. If there is no moderator, you
could take the same steps for dealing with news as are recommended
for mail with all the same caveats.

A reasonable printed reference one might obtain has been published by
O'Reilly and Associates, _Stopping Spam_, by Alan Schwartz and Simson
Garfinkel [4]. This book also has interesting histories of spammers
such as Cantor and Siegel, and Jeff Slaton. It gives fairly clear
instructions for filtering mail and news.

5. Help for Beleaguered Admins

As a system administrator, news administrator, local Postmaster, or
mailing-list administrator, your users will come to you for help in
dealing with unwanted mail and posts. First, find out what your
institution's policy is regarding unwanted/unsolicited mail. It is
possible that it won't do anything for you, but it is also possible
to use it to justify blocking a domain which is sending particularly
offensive mail to your users. If you don't have a clear policy, it
would be really useful to create one. If you are a mailing-list
administrator, make sure your mailing-list charter forbids off-topic
posts. If your internal-only newsgroups are getting spammed from the
outside of your institution, you probably have bigger security
problems than just spam.

Make sure that your mail and news transports are configured to reject
messages injected by parties outside your domain. Recently
misconfigured Netnews servers have become subject to hijacking by
spammers. SMTP source routing <@relay.host:user@dest.host> is
becoming deprecated due to its overwhelming abuse by spammers. You
should configure your mail transport to reject relayed messages (when
neither the sender nor the recipient are within your domain). Check:

http://www.sendmail.org/

under the "Anti-Spam" heading.

If you run a firewall at your site, it can be configured in ways to
discourage spam. For example, if your firewall is a gateway host
that itself contains an NNTP server, ensure that it is configured so
it does not allow access from external sites except your news feeds.
If your firewall acts as a proxy for an external news-server, ensure
that it does not accept NNTP connections other than from your
internal network. Both these potential holes have recently been
exploited by spammers. Ensure that email messages generated within
your domain have proper identity information in the headers, and that
users cannot forge headers. Be sure your headers have all the
correct information as stipulated by RFC 822 [5] and RFC 1123 [6].

If you are running a mailing-list, allowing postings only by
subscribers means a spammer would actually have to join your list
before sending spam messages, which is unlikely. Make sure your
charter forbids any off-topic posts. There is another spam-related
problem with mailing-lists which is that spammers like to retaliate
on those who work against them by mass-subscribing their enemies to
mailing-lists. Your mailing-list software should require
confirmation of the subscription, and only then should the address be
subscribed.

It is possible, if you are running a mail transfer agent that allows
it, to block persistant offending sites from ever getting mail into
your site. However, careful consideration should be taken before
taking that step. For example, be careful not to block out sites for
which you run MX records! In the long run, it may be most useful to

help your users learn enough about their mailers so that they can
write rules to filter their own mail, or provide rules and kill files
for them to use, if they so choose.

There is information about how to configure sendmail available at
"www.sendmail.org." Help is also available at "spam.abuse.net."

Another good strategy is to use Internet tools such as whois and
traceroute to find which ISP is serving your problem site. Notify
the postmaster or abuse (abuse@offending-domain.example) address that
they have an offender. Be sure to pass on all header information in
your messages to help them with tracking down the offender. If they
have a policy against using their service to post unsolicited mail
they will need more than just your say-so that there is a problem.
Also, the "originating" site may be a victim of the offender as well.
It's not unknown for those sending this kind of mail to bounce their
mail through dial-up accounts, or off unprotected mail servers at
other sites. Use caution and courtesy in your approach to those who
look like the offender.

News spammers use similar techniques for sending spam to the groups.
They have been known to forge headers and bounce posts off "open"
news machines and remailers to cover their tracks. During the height
of the infamous David Rhodes "Make Money Fast" posts, it was not
unheard of for students to walk away from terminals which were logged
in, and for sneaky folks to then use their accounts to forge posts,
much to the later embarrassment of both the student and the
institution.

One way to lessen problems is to avoid using mail-to URLs on your web
pages. They allow email addresses to be easily harvested by those
institutions grabbing email addresses off the web. If you need to
have an email address prevalent on a web page, consider using a cgi
script to generate the mailto address.

Participate in mailing lists and news groups which discuss
unsolicited mail/posts and the problems associated with it.
News.admin.net-abuse.misc is probably the most well-known of these.

6. What's an ISP to Do

As an Internet Service Provider, you first and foremost should decide
what your stance against unsolicited mail and posts will be. If you
decide not to tolerate unsolicited mail, write a clear Acceptable Use
Policy which states your position and delineates consequences for
abuse. If you state that you will not tolerate use of your resource
for unsolicited mail/posts, and that the consequence will be loss of
service, you should be able to cancel offending accounts relatively
quickly (after verifying that the account really IS being mis-used).
If you have downstreaming arrangements with other providers, you
should make sure they are aware of any policy you set. Likewise, you
should be aware of your upstream providers' policies.

Consider limiting access for dialup accounts so they cannot be used
by those who spew. Make sure your mail servers aren't open for mail
to be bounced off them (except for legitimate users). Make sure your
mail transfer agents are the most up-to-date version (which pass
security audits) of the software.

Educate your users about how to react to spew and spewers. Make sure
instructions for writing rules for mailers are clear and available.
Support their efforts to deal with unwanted mail at the local level -
taking some of the burden from your system administrators.

Make sure you have an address for abuse complaints. If complainers
can routinely send mail to "abuse@BigISP.example" and you have
someone assigned to read that mail, workflow will be much smoother.
Don't require people complaining about spam to use some unique local
address for complaints. Read and use 'postmaster' and 'abuse'. We
recommend adherence to RFC 2142, _Mailbox Names for Common Services,
Roles and Functions._ [7].

Finally, write your contracts and terms and conditions in such
language that allows you to suspend service for offenders, and so
that you can impose a charge on them for your costs in handling the
complaints their abuse generates and/or terminating their account and
cleaning up the mess they make. Some large ISPs have found that they
can fund much of their abuse prevention staff by imposing such
charges. Make sure all your customers sign the agreement before
their accounts are activated. There is a list of "good" Acceptable
Use Policies and Terms of Service at:

http://spam.abuse.net/goodsites/index.html.

Legally, you may be able to stop spammers and spam relayers, but this
is certainly dependent on the jurisdictions involved. Potentially,
the passing of spam via third party computers, especially if the
headers are forged, could be a criminal action depending on the laws
of the particular jurisdiction(s) involved. If your site is being
used as a spam relay, be sure to contact local and national criminal
law enforcement agencies. Site operators may also want to consider
bringing civil actions against the spammer for expropriation of
property, in particular the computer time and network bandwidth. In
addition, when a mailing list is involved, there is a potential
intellectual property rights violation.

There are a few law suits in the courts now which claim spammers
interfered with and endangered network connectivity. At least one
company is attempting to charge spammers for the use of its networks
(www.kclink.com/spam/).

7. Security Considerations

Certain actions to stop spamming may cause problems to legitimate
users of the net. There is a risk that filters to stop spamming will
unintentionally stop legitimate mail too. Overloading postmasters
with complaints about spamming may cause trouble to the wrong person,
someone who is not responsible for and cannot do anything to avoid
the spamming activity, or it may cause trouble out of proportion to
the abuse you are complaining about. Be sure to exercise discretion
and good judgment in all these cases. Check your local escalation
procedure. The Site Security Handbook [2] can help define an
escalation procedure if your site does not have one defined.

Lower levels of network security interact with the ability to trace
spam via logs or message headers. Measures to stop various sorts of
DNS and IP spoofing can make this information more reliable.
Spammers can and will exploit obvious security weaknesses, especially
in NNTP servers. This can lead to denial of service, either from the
sheer volume of posts, or as a result of action taken by upstream
providers.

8. Acknowledgments

Thanks for help from the IETF-RUN working group, and also to all the
spew-fighters. Specific thanks are due to J.D. Falk, whose very
helpful Anti-spam FAQ proved valuable. Thanks are also due to the
vigilance of Scott Hazen Mueller and Paul Vixie, who run
spam.abuse.net, the Anti-spam web site. Thanks also to Jacob Palme,
Chip Rosenthal, Karl Auerbach for specific text: Jacob for the
Security Considerations section, Chip for the configuration
suggestions in section 5, Karl for the legal considerations. Andrew
Gierth was very helpful with Netnews spam considerations. And thanks
to Gary Malkin for proofing and formatting.

Spam is a name of a meat product made by Hormel. "spam" (no
capitalization) is routinely used to describe unsolicited bulk
email and netnews posts.

10. Appendix - How to Track Down Spammers

In a large proportion of spams today, complaining to the postmaster
of the site that is the apparent sender of a message will have little
effect because either the headers are forged to disguise the source
of the message, or the senders of the message run their own
system/domain, or both.

As a result, it may be necessary to look carefully at the headers of
a message to see what parts are most reliable, and/or to complain to
the second or third-level Internet providers who provide Internet
service to a problem domain.

In many cases, getting reports with full headers from various
recipients of a spam can help locate the source. In extreme cases of
header forgery, only examination of logs on multiple systems can
trace the source of a message.

With only one message in hand, one has to make an educated guess as
to the source. The following are only rough guidelines.

In the case of mail messages, "Received:" headers added by systems
under control of the destination organization are most likely to be
reliable. You can't trust what the source domain calls itself, but
you can usually use the source IP address since that is determined by
the destination domain's server.

In naive mail forgeries, the "Message-ID:" header may show the first
SMTP server to handle the message and/or the "Received:" headers may
all be accurate, but neither can be relied on. Be especially wary
when the Received: headers have other headers intermixed. Normally,
Received: headers are all together in a block, and when split up, one
or the other blocks is probably forged.

In the case of news messages, some part of the Path: header may be a
forgery; only reports from multiple sites can make this clear. In
naive news forgeries, the "NNTP-Posting-Host:" header shows the
actual source, but this can be forged too.

If a spam message advertises an Internet server like a WWW site, that
server must be connected to the network to be usable. Therefore that
address can be traced. It is appropriate to complain to the ISP
hosting a web site advertised in a SPAM, even if the origin of the
spam seems to be elsewhere. Be aware that the spam could be an
attack on the advertised site; the perpetrator knows the site will be
deluged with complaints and their reputation will be damaged. Any
spam with an electronic address in it is suspect because most
spammers know they're unwelcome and won't make themselves accessible.

Doing a traceroute on an IP address or DNS address will show what
domains provide IP connectivity from you to that address.

Using whois and nslookup, one can try to determine who is
administratively responsible for a domain.

In simple cases, a user of a responsible site may be exploiting an
account or a weakness in dial-up security; in those cases a complaint
to a single site may be sufficient. However, it may be appropriate to
complain to more than one domain, especially when it looks like the
spammers run their own system.

If you look at the traceroute to an address, you will normally see a
series of domains between you and that address, with one or more
wide-area/national Internet Service Providers in the middle and
"smaller" networks/domains on either end. It may be appropriate to
complain to the domains nearer the source, up to and including the
closest wide-area ISP. However, this is a judgement call.

If an intermediate site appears to be a known, responsible domain,
stopping your complaints at this point makes sense.

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